Can't sculpt Meta Ball
You can only sculpt mesh objects. Right now you have a meta object. You need to convert it to a mesh first.

Do note that after converting to a mesh you will not be able to edit it with the meta objects any longer (so you might want to duplicate the whole thing and move it to another layer first).

You can easily convert it to a mesh by selecting one of your meta balls, pressing AltC then choosing Mesh from Curve/Meta/Surf/Text.

Or from the 3D view header Object > Convert to > Mesh from Curve/Meta/Surf/Text